Many industries are dependent on instruments that measure the properties of materials. Improved measurements often result in improved efficiency, as well as improved product quality.
For example, electric power plants burning coal must know the heating value, moisture content, ash, and sulfur in the coal they use. Coal used for liquefaction or other processing must be measured for additional properties. In the food processing industry, moisture, oil content, density, color, and other parameters are of importance in the production of baked goods, frozen foods, dairy products, cereals, and others. Moisture in wood and its derivatives is critical to lumber, paper, and related industries. Cement must have no more than a specific limit in moisture content when produced, and moisture measurements in cement during preparation and curing are often critical for sufficient durability of the final product. The chemical industry is particularly dependent on material property measurements, including polymerization, viscosity, color, transparency, and other determinations. Petroleum refining is similarly dependent on many property measurements, including heating value, hydrogen to carbon ratio, water content and others. Prospecting often requires measurements of porosity of drilling cores, kerogen content, water, and others. Clearly, the reliance of material property measurements in industry is extensive and profoundly important.
Material properties are traditionally measured by some form of invasive or destructive technique (such as weight loss or gain, combustion, combination with reagents, and the like) but can often be measured by means of electromagnetic radiation absorbed or emitted by the material. All of the critical measurements listed in the previous paragraph can be made by means of some form of electromagnetic radiation. Electromagnetic radiation measurements have the advantage that they are generally nondestructive to the sample and can be performed without making direct contact with it, other than any contact with the sample container necessary to position the sample under the appropriate sensors.
The electromagnetic spectrum spans many orders of magnitude in wavelength. Microwaves and radio waves have the longest wavelengths (millimeters to meters in wavelength), followed by infrared (micrometers), visible light (parts of a micrometer), ultraviolet (manometers), x-rays and gamma rays (sub-manometers).
Each wavelength band of the electromagnetic spectrum can be used to measure properties of materials. For example, structural and quantitative analytical determinations of material properties may be obtained. Table 1 shows the chemical information provided by the different principal regions of the electromagnetic spectrum. Radio waves can interact with the nuclei of the material in nuclear magnetic resonance (NMR), revealing molecular mobility and isotopic content. Microvaves can be selectively absorbed or phase shifted by water and other dielectrics. Infrared can be selectively absorbed by specific molecular structures. Visible light and ultraviolet are absorbed or reflected by specific atomic and molecular species in the material. X-rays and gamma rays reveal atomic and isotopic composition. Any of these wavelength bands may further be used for imaging of the sample material.
TABLE 1 ______________________________________ ELECTROMAGNETIC SPECTRUM CHEMICAL INFORMATION Wavelength Wave Number Region Range Range Transition ______________________________________ Gamma &lt;10 pm &gt;10.sup.9 cm.sup.-1 Nuclear Rays X-rays 10 pm-10 nm 10.sup.9 -10.sup.6 cm.sup.-1 K and L electron Ultraviolet 10-400 nm 10.sup.6 -2.5 .times. 10.sup.4 cm.sup.-1 Valence electron Visible 400-800 nm 2.5 .times. 10.sup.4 -1.25 .times. 10.sup.4 Valence cm.sup.-1 electron Infrared 800 nm-1 mm 1.25 .times. 10.sup.4 -10 cm.sup.-1 Molecular Microwave 1 mm-100 mm 10-0.1 cm.sup.-1 Rotations Radio- &gt;100 mm &lt;0.1 cm.sup.-1 Spin frequency ______________________________________
Instruments that detect electromagnetic radiation may be categorized according to the type of interaction between the sample material and electromagnetic radiation. In the case of infrared absorption measurements, for example, the electromagnetic radiation that passes through the sample and is detected, the remainder being absorbed or scattered. In the case of color determinations, visible light is reflected from the sample in certain wavelength regions and is detected. In the case of NMR, radio signals are absorbed resonantly by precessing nuclei in a magnetic field as the nuclei change their spin direction. Radio signals are subsequently emitted by the nuclei as they return to their initial orientation. Emission measurements, such as radiative capture of neutrons to produce characteristic gamma rays, involve the production of detectable electromagnetic radiation following a non-electromagnetic stimulus.
Prior art instruments that measure material properties by detecting electromagnetic radiation have concentrated on one or a few wavelength bands and generally rely on one type of measurement. For example, infrared absorption or reflectance measurements are related to only a subset of the important material properties, while visible and ultraviolet spectrometers measure another set, and x-ray and gamma detectors yet another set of material properties, each sensing and detecting technique being restricted to specific wavelength limits. Such separate measurement processes are inefficient for inhomogeneous materials because multiple separate samples must normally be obtained for each instrument to create a representative data set.
An additional disadvantage of prior art instruments is that the data obtained from such fragmented diagnostics often has limited utility until it can be combined with measurements from other measurement modes. This is particularly the case for complex manufacturing processes in which control is exercised most effectively when data from diverse measurement means are combined to produce an overall status evaluation of the process plant.
The fusion of data from diverse measurement techniques can provide additional information about the material not measured but inferred from the combination of measurements, involving conservation laws and other principles. However, data fusion is generally labor intensive, since the instruments providing measurements by different modalities are usually difficult to adapt to each other. The data, and the electronic or physical forms of the data, are often incompatible, requiring manual translation and analysis.
To consider a specific example, coal properties of interest to many users are the moisture content, hydrogen content not bound in water,, heating value, and sulfur in various forms. The hydrocarbon molecular forms and some sulfur compounds can be measured by infrared spectroscopy, the interstitial water can be seen with microwaves, and other compounds can be detected by visible and ultraviolet spectroscopy. These tests are generally performed using separate instruments, and combined to determine the coal properties.